Fuel cells have been proposed as a power source for many applications. One such fuel cell is the proton exchange membrane or PEM fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a so-called “membrane-electrode-assembly” or MEA comprising a thin, proton-conductive, polymeric, membrane-electrolyte having an anode electrode film formed on one face thereof, and a cathode electrode film formed on the opposite face thereof. Such membrane-electrolytes are well known in the art and are described in such as U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia.
In general, such membrane-electrolytes are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION3 available from the E. I. DuPont de Nemeours & Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material such as NAFION3 intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the present invention.
The MEA is sandwiched between sheets of porous, gas-permeable, conductive material which press against the anode and cathode faces of the MEA and serve as (1) the primary current collectors for the anode and cathode, and (2) mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh noble metal screen, and the like, as is well known in the art. This assembly is referred to as the MEA/primary current collector assembly herein.
The MEA/primary current collector assembly is pressed between a pair of non-porous, electrically conductive plates or metal sheets which serve as secondary current collectors for collecting the current from the primary current collectors and conducting current between adjacent cells internally of the stack (i.e., in the case of bipolar plates) and at the ends of a cell externally of the stack (i.e., in the case of monopolar plates). The secondary current collecting plate contains a flow field that distributes the gaseous reactants (e.g., H2 and O2/air) over the surfaces of the anode and cathode. These flow fields generally include a plurality of lands which engage the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply header at one end of the channel and an exhaust header at the other end of the channel.
Conventionally, these metal plates have a single functional flow field defining a particular geometry of the flow channel. One generally known flow field defines serpentine flow channels which connect the supply and exhaust header after making a number of hairpin turns and switch backs. Serpentine flow channels thus define a contiguous, albeit tortuous flow path. Another generally known flow field defines interdigitated flow channels in which a plurality of flow channels extending from the supply header towards the exhaust header but terminating at deadends are interdigitated between a plurality of flow channels extending from the exhaust header towards the supply header but terminating at deadends. In contrast to serpentine flow channels, these interdigitated flow channels define a noncontiguous path such that flow between the supply and exhaust header is achieved when the gaseous reactants traverses a land between adjacent flow channels through the porous primary current collector.
Conventionally, a bipolar plate is formed by assembling a pair of metal sheets such that a functional flow field is formed on each side of the bipolar plate assembly. Often times a spacer is interdisposed between the metal sheets to define an interior volume to permit coolant flow through the bipolar plate assembly. One such bipolar plate assembly is described in U.S. Pat. No. 5,776, 624 issued Jul. 7, 1998, and assigned to the assignee of the present invention.